1. Field of the Invention:
This invention relates generally to plastic and non-metallic article shaping or treating processes and particularly to reshaping running or indefinite length work; specifically, the invention relates to improved methods and apparatus for producing double-face corrugated paperboard webs formed by laminating flat facing webs to opposite sides of a corrugated paper web.
2. Description of the Prior Art:
Corrugated paperboard is manufactured at high production rates on corrugator machines which are well known in the paper industry. A typical machine includes a corrugating and gluing section, a heating section, and a cooling section. In the first section, corrugations are formed transversely across an intermediate web and liquid adhesive is applied to the tips of the flutes of the corrugated web or medium. After the adhesive is applied, a first single-face liner web is brought into contact with the glue-coated flutes to form a laminated single-face web consisting of one liner and the corrugated medium. The single-face web is then advanced past a glue machine downstream to apply adhesive to the exposed flute tips of the medium and thereafter a second double-face liner web is applied to the exposed side of the corrugated medium. The combined double-face web consisting of a single face web and the second liner then passes through a heating section where the liquid adhesive holding the second liner to the corrugated medium is cured. The adhesive is cured by passing the freshly glued web across a series of hotplates under pressure from above. The hotplates are usually heated internally by steam to a temperature needed to cure the adhesive. The pressure is provided by moving the web over the hotplates under an endless ballast belt which rests upon the upper liner of the single-face web and advances together with the web at the same speed. Weight rollers on top of the lower flight of the belt provide additional pressure to hold the web lamina together and maintain them flat against the hotplates to enhance heat transfer from the hotplates to the web to cure the adhesive. As the heat acts upon the adhesive, it also drives moisture out of the combined web so that the finished corrugated paperboard web exists from the downstream end of the heating section in a stiff and substantially flat condition. The web then passes immediately through a cooling section to reduce its temperature prior to being divided into a plurality of webs of selected widths each of which is then cut transversely to form corrugated paperboard blanks.
One particular difficulty that has plagued the corrugated paperboard industry for many years is that the finished blanks tend to be warped in one or more directions which makes it difficult to form them into containers. This tendency has been attributed at various times to different production factors such as residual stresses, moisture variations, adhesive quantity, induced tension, and heat transfer characteristics. Many corrective methods and apparatus have been used with limited degrees of success.
It is helpful to understand that a warped blank is not flat; instead, it may be curled slightly upward or downward on both sides across the width of the machine (hereinafter called cross-machine direction or C-D warp); it may be curled slightly upward or downward on both ends in its direction of travel (hereinafter called machine-direction or M-D warp); it may be curled upward on one side and downward on the other across the width of the machine (hereinafter called S-warp); or diagonal corners of the blank may curl upward or downward in the same direction (hereinafter called twist-warp).
Much attention has been given to the application of heat to the combined web in the heating section to improve overall quality and reduce warp. Exemplary patents include: Cassady U.S. Pat. No. 2,941,573 showing movable hotplates to control heat transfer by selectively spacing the plates from the web; Moser et al U.S. Pat. No. 2,993,527 showing pressure-loaded rolls to maintain bonding pressure against the web; Moser et al U.S. Pat. No. 3,226,840 showing an air-film system to selectively reduce heat transfer; Shields U.S. Pat. No. 3,472,158 showing application of weight rollers to increase bonding pressure; Nitchie U.S. Pat. No. 3,175,300 showing another air-film system to selectively reduce heat transfer; Stewart U.S. Pat. No. 3,347,732 showing the application of air pressure to the top of the web to hold it against the hotplates; and Hayasi et al U.S. Pat. No. 3,829,338 showing a temperature feedback system for varying the effective weight of ballast rollers to control heat transfer.
Other factors are also known to influence the tendency of the blanks to warp. For example, moisture imbalances between the single-face and double-face liner are known to create internal stresses in the web which results in warp in the blanks made from the web. The initial addition of moisture is made in the form of steam to the corrugated medium supply web; suitable apparatus for applying steam is illustrated din Bruker U.S. Pat. No. 2,674,299 and Bruker et al U.S. Pat. No. 2,718,712. In addition, water vapor may be applied to the double-face liner such as shown in Gebbie U.S. Pat. No. 2,987,105 and, for that matter, in a similar manner to the single-face web.
Another factor contributing to the moisture content of the various lamina is the adhesive to bond the lamina together. The adhesive commonly used is an ungelatinized granular starch in a liquid carrier that is cured by gelatinization and dehydration which result from the application of heat. Apparatus commonly used for applying the adhesive to the tips of the exposed flutes of the single-face web is shown in Thorn U.S. Pat. No. 2,827,873; similar apparatus is also used to apply the adhesive to the flute tips of the corrugated medium just prior to joining the medium to the single-face liner.
Still another factor influencing warp is the amount of heat applied to the various lamina beforoe they are joined as well as heat applied to the single-face web and double-face liner before these are joined. The application of moisture and heat is normally referred to as preconditioning and results in dimensional changes in the lamina. The application of moisture may be made with the apparatus mentioned above; heat may be applied by warping the webs around a large heated drum. The amount of heat applied at a given speed can be controlled by varying the distance that the web is warped around the drum. Examples are shown in Bruker U.S. Pat. No. 2,710,045 and Sherman U.S. Pat. No. 3,218,219.
One final factor that affects warp, especially M-D warp in the direction of web travel is the tension applied to both the single-face web and the double-face liner prior to their being adhesively joined. Such tension may be applied to the single-face web by, for example, a vacuum device such as shown in Shield's U.S. Pat. No. 3,438,449 or a vacuum device such as shown in Middleman U.S. Pat. No. 3,788,515. Tension may be applied to the double-face liner by a device such as shown in Drenning U.S. Pat. No. 3,257,086 or even by a dancer roll pressed against the liner such as shown in Sherman U.S. Pat. No. 3,218,219 or in any similar manner.
In the past, the moisture content in the various lamina, the amount of adhesive applied to the flutes of the medium, the amount of heat applied to the lamina and the amount applied to cure the adhesive, and the tension applied to the single-face web and double-face liner have been individually and manually controlled according to the skills of the production operator. But, even with a high degree of skill, it is almost impossible for an operator to adjust all the variables to the extent necessary to consistently produce warp-free blanks, particularly since the variables are interrelated such that adjustment of one variable may often nullify or at least seriously affect the adjustment of another variable. In addition, the adjustment of most of the variables is dependent on the speed of production and, to further complicate matters, the dependecy is not directly linear.
The result of incorrect adjustment or failure to adjust certain variables usually results in the production of inferior blanks and often a great deal of scrap corrugated web, particularly the web produced in the interim between adjustments required because of changes in production speed, such speed often being changed because of the nature of production of corrugated paperboard webs and blanks.
Now, after considerable study and testing, it has been found that no single production factor can exclusively control warp occuring in the finished blanks. Rather, it is a combination of factors which, when controlled in accordance with this invention, results in warp-free blanks or at least results in a considerable reduction of warp.
The method of this invention may be performed with substantially conventional apparatus modified to the extent necessary to provide for adjustment by the control system of the present invention. However, operation is improved by the use of apparatus improved to include additional functions and capabilities as will be hereinafter described.